The present invention pertains to the field of tuning systems with automatic fine tuning provisions.
Automatic fine tuning (AFT) arrangements are widely employed in tuning systems for radio and television receivers to correct for errors of the frequency of the local oscillator signal due to variations of the characteristics of components within the tuning system. However, tuning systems which are not particularly susceptible to frequency errors due to component variations can also employ AFT arrangements to correct for frequency offsets in the received signal which may be introduced, for example, by frequency conversion apparatus utilized with cable and master antenna installations. Thus, tuning systems which include a phase locked loop (PLL) for synthesizing a local oscillator signal from a relatively stable frequency reference signal generated by a crystal oscillator arrangement often also include an AFT arrangement which is selectively enabled to control the frequency of the local oscillator signal after the PLL has caused the local oscillator signal to have a nominal frequency associated with a selected channel to correct for frequency offsets of the respective RF signal.
Since PLLs for the most part comprise digital circuits and AFT arrangements for the most part comprise analog circuits, respective integrated circuit technologies may not be compatible with each other. As a result, the incorporation of a significant portion of a tuning system including a PLL and an AFT arrangement into a single integrated circuit is hindered. So that a PLL tuning system and AFT provisions for it may be incorporated in a single integrated circuit, the analog AFT arrangement may be replaced by a second phase locked loop configuration for reducing the error between the actual frequency of an IF signal and its desired value due to a frequency offset of a respective RF signal. However, a PLL configuration for the AFT function, because of its digital nature, tends to be more susceptible than analog AFT arrangements to peculiar RF signal conditions such as, e.g., the overmodulation of the RF signal, than analog AFT arrangements. In addition, such PLL AFT configurations may require modification of conventional IF circuits to provide the type of signals which are processed in PLLs. Therefore, there is a need for AFT arrangements which while analog in nature are compatible with integrated circuit technologies for PLLs.
While analog AFT arrangements have advantages over digital AFT arrangements in PLL tuning systems, they tend to be more affected by variations in the gain versus frequency characteristics of the tuning system. Variations of the gain versus frequency characteristics result for the most part because the voltage controlled oscillators typically employed in such tuning systems are appreciably more sensitive to changes of the control voltage in the UHF range than in the VHF range. To account for the relative insensitivity of the local oscillator to changes of the control voltage in the VHF range and still ensure a reasonable response time in the VHF range, time constants associated with the tuning system may be selected to be relatively short. While such selections satisfy the response time requirements in the VHF range, they tend to increase the gain of the tuning system in the UHF range, where the gain is already relatively high, and thereby tend to make the operation of the tuning system in the UHF range approach instability. Therefore, there is a need for apparatus for effectively controlling the sensitivity of a tuning system including an analog AFT arrangement in accordance with the frequency band in which a selected channel resides.